Data transmission method and system

ABSTRACT

In a data transmission method and system, user signals are transmitted wirelessly, and a maximum delay for the system has been defined. The system includes circuitry at the transmitting end for sequentially performing an outer and inner coding on the signals to be transmitted, and circuitry for performing a first interleaving after the outer coding. The interleaving length of the first interleaving is chosen within the scope of the maximum delay defined for the system. To ensure improved performance, the system includes circuitry for performing a second interleaving after the inner coding, and the interleaving length of the second interleaving is substantially equal to the interleaving length of the first interleaving.

FIELD OF THE INVENTION

The invention relates to a data transmission method in a wirelesstelecommunications system wherein the user signals are transmitted in agiven channel and a maximum delay has been defined for said system, andwherein an outer coding and an inner coding are sequentially performedto the signals to be transmitted, and wherein a first interleaving isperformed to a signal after the outer coding, the interleaving length ofsaid first interleaving being chosen within the scope of the maximumdelay defined for the system.

BACKGROUND OF THE INVENTION

As well known, the channel used for transmitting signals intelecommunications connections causes interference intelecommunications. This occurs independently of the physical form ofthe channel, whether the channel is a radio connection, an optical fibreor a copper cable, for instance.

In order to reduce the channel-caused interference and its effects andto ensure a more reliable transmission connection, a digital signal iscoded. A typical coding method used in cellular radio applications isconvolution coding, which is well suited to a fading channel. In atypical radio channel between a base station and a mobile station,errors occur in bursts, in other words they are unevenly distributed intime domain. This is the reason for using in cellular radio systemsinterleaving, which aims at changing the bursty errors caused by achannel to separate errors independent of each other. In a knowninterleaving system used in the CDMA systems, for example, the codedbinary symbols are grouped into blocks in which the order of the bits ischanged before transmission. This method can be called bit interleaving.

Wireless data transmission systems have conventionally been used only totransmit speech. The growth in the number of different services to betransmitted means, particularly in wireless systems, that the systemmust be able to transmit different capacity signals, such as speech ,for instance, at a data rate of 8 kbit/s and data at a rate of 64kbit/s, over a radio path. The data transmission system should thus beable to operate effectively in an environment where transmissions ofseveral different data rates, quality standard requirements and servicetypes are transmitted. Services of different type tolerate delays ofdifferent length, and maximum permissible delays corresponding to aparticular service are set for the system, and the interleaving, forinstance, also has to be performed within the scope of the maximumdelays.

To enable transmission of high data rate signals particularly, codingmethods have been developed in which two sequential codings areperformed, a so called outer coding first, and a so called inner codingnext. As a typical outer coding can be Reed-Solomon coding, forinstance, whereupon a signal is usually formed to be frame-structured,and as a typical inner coding can be some convolution coding. Owing tothe above reasons, interleaving has previously been employed inconnection with this method. Interleaving has been performed in such amanner that the interleaving length of the interleaving performed afterthe outer coding has been chosen on the basis of the maximum systemdelay. The interleaving length of the interleaving subsequent to theinner coding has been determined by the length of the frame structure.This has aimed at minimizing the system delay.

The problem in the above arrangement is that the benefit gained frominterleaving cannot be utilized in full, since the interleaving lengthof the interleaving has been determined by the length of the framestructure, which typically is substantially shorter than the maximumsystem delay.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is thus to provide a method and a systemsolving the above problem. This is achieved with a method described inthe introduction, the method being characterized in that a secondinterleaving is performed to the signal after the inner coding, theinterleaving length of said second interleaving being substantiallyequal to the interleaving length of the first interleaving.

The invention further relates to a data transmission system wherein usersignals are transmitted wirelessly in a given channel and a maximumdelay has been defined for said system, said system comprising means atthe transmitting end for performing sequentially an outer coding and aninner coding to the signals to be transmitted, means for performing afirst interleaving to a signal after the outer coding, the interleavinglength of said first interleaving being chosen within the scope of themaximum delay determined for the system. The system of the invention ischaracterized in that the system comprises means for performing a secondinterleaving to the signal after the inner coding, the interleavinglength of said second interleaving being substantially equal to theinterleaving length of the first interleaving.

Several advantages are achieved by the method and system of theinvention. The invention is based on the fact that the interleavingsubsequent to the inner coding does not have to be dependent on thelength of the system frame structure, but the interleaving can be equalin length to the interleaving subsequent to the outer coding. In thesolution of the invention, the system performance increases on accountof the improved time diversity, which is important particularly whenhigh capacity connections are involved. If a maximum system delay is 220ms, for example, and the length of the system frame is 10 ms, theinterleaving subsequent to the outer coding has previously beenperformed at a 100 ms interleaving length and the interleavingsubsequent to the inner coding has been performed at a 10 msinterleaving length. The total delay caused by interleaving is thus 100ms+10 ms, i.e., 110 ms, which leaves 20 ms for the other delays. In thesolution of the invention, each interleaving is performed at a 100 msinterleaving length, whereby the time diversity of the interleaving canbe utilized considerably more than before. Contrary to the prior art, inthe solution of the invention the total system delay does not increaseeven though the interleaving length is increased.

An essential advantage of the invention is also that the total systemdelay can be decreased by the solution of the invention withoutimpairing the performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail in connection with thepreferred embodiments with reference to the accompanying drawings, inwhich

FIG. 1 shows an example of the data transmission system of theinvention,

FIG. 2 illustrates a possible structure of the transmitter of theinvention by means of a block diagram,

FIG. 3 illustrates another possible structure of the transmitter of theinvention by means of a block diagram.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a wireless data transmission system, in other wordsthe part of a cellular radio system in which the method of the inventioncan be applied. The system comprises a base station 100, which is inbi-directional connection 102 to 106 to subscriber terminals 108 to 112.In the cellular radio system used as an example, the CDMA system isemployed, and when being applied, the invention will be described belowwithin the CDMA system without being restricted to it, however, which isunderstood by those skilled in the art. The essential features of theinvention are independent of the multiple access system used.

In the system in FIG. 1, each connection has typically a separatespreading code whose bit rate, in other words a so-called chip rate, issubstantially higher than the data rate, and by which the data to betransmitted has been multiplied and thus spread to a wide bandwidth.Receivers are able to distinguish the desired signal from other signalstransmitted in the same frequency band on the basis of the spreadingcode. The solution of the invention to implement interleaving can beutilized both in a terminal equipment and in a base station.

The operation of the solution of the invention will be illustrated inthe following by means of the block diagram of FIG. 2. The block diagramshows a simplified structure of the transmitter of the system of theinvention. Only components of the apparatus essential for the inventionare shown. A signal 200 to be transmitted is applied to means 202performing an outer coding. Reed-Solomon type coding can be preferablyused as the outer coding, since it is well suited to correcting burstyerrors. The coded signal is applied to first interleaving means 204wherein a first interleaving is performed to the signal, theinterleaving length of said first interleaving being chosen on the basisof the maximum delay defined for the system. The signal interleaved insuch a manner is further applied to means 206 which perform an innercoding, and in which convolution coding can preferably be used. Thecoding method is not essential, however, as far as the present inventionis concerned.

After the inner coding, the signal is applied to means 208 wherein asecond interleaving is performed, whose interleaving length issubstantially equal to the interleaving length of the firstinterleaving. The interleaving and coding means can preferably beimplemented by signal or general processors or by separate components,as is understood by those skilled in the art. After the secondinterleaving the signal is applied to a modulator 210 wherein the signalis modulated in accordance with the prior art, and where a spreadspectrum system is concerned, the signal is multiplied before themodulation by a spreading code characteristic of the connection. Themodulated signal is applied to an antenna 214 through radio frequencyparts 212.

Let us next take a closer look at the preferred structure of thetransmitter of the data transmission system of the invention by means ofthe block diagram of FIG. 3. The transmitter of the invention comprisesa number of data transmitters 356 to 360 whose output signals 300 to 304can have different quality standard requirements and data rates, andsaid data sources can generate signals to be transmitted simultaneously.Typical data transmitters that may come into question are, for example,a speech coder, telefax, videocoder or another source which generatesdigital data.

The transmitter of the invention further comprises first means 306, 308for performing an outer coding to at least some signals to betransmitted in such a manner that on account of the coding, the qualitystandard requirement of the signals coded in this manner is as high asthe lowest quality standard requirement of the active data sources. Thesame quality standard requirement is thus provided for all signals to betransmitted. The coding used can be performed in known manners, and theReed-Solomon coding system can preferably be used. The transmitterfurther comprises first means 310, 312 for interleaving the codedsignals, in other words for modifying the locations of the signalsymbols in accordance with a known algorithm, using the interleavinglength chosen on the basis of the maximum delay defined for the system.

Signals 300, 316, 318 obtained from all data sources and having a commonquality standard requirement are applied to a multiplexer 314 whereinthe signals are combined in time domain into frames of equal length, thenumber of bits in said frames being variable frame by frame, dependingon the signals arriving at the input of the multiplexer. The output ofthe multiplexer is operatively connected to second coding means 320wherein an inner coding is performed to a signal 348. This coder 320 maybe any desired coder; in the preferred embodiment is used a convolutioncoder by which the common quality standard requirements can be lowered.

The signal 350 coded in this manner is further applied to means 322wherein the number of symbols in each frame to be transmitted isequalized in such a manner that the length of the symbols to betransmitted in each frame is equal to the multiple of the known timeperiod which is substantially shorter that the symbol length. When theCDMA method is used, the multiple of the length of a spreading code bit,in other words chip, may be in question.

The equalization of the number of the symbols can be performed either byrepeating or removing symbols, both of which methods can be implementedby means of signal processing, as is well understood by those skilled inthe art.

The transmitter of the system of the invention further comprises means334 for combining data 326 on the frame structure of each frame, such asbit rate, to the symbol-equalized signal frames 324 to be transmitted.Said data 326 on the frame structure is first applied to coding means332 wherein the desired coding to ensure the protection of the dataagainst transmission errors is performed, and said coded signal isapplied to a multiplexer 338. Said data may comprise, for example, saidframe or a description of the frame structure of the frame to betransmitted next. The signal 352 combined in this manner is furtherapplied to second interleaving means 336. In the interleaving means theinterleaving is performed using substantially equal interleaving lengthas in connection with the first interleaving.

The transmitter may comprise the multiplexer 338 wherein additionaldata, such as power regulation data 330 or reference symbols 328, iscombined to the interleaved signal for coherent reception. When a CDMAtransmitter is involved, the combined signal 340 is further applied to ameans 342 wherein the signal is multiplied by a spreading codecharacteristic of the connection, whereby the signal spreads to thefrequency band used. The signal multiplied in this manner is furtherapplied to radio frequency parts wherein the signal is converted to theradio frequency and amplified to be transmitted by an antenna 346.

The transmitter of the system of the invention further comprises controlmeans 354 controlling the operation of the above parts, and said meanscan be implemented by means of a microprocessor or control logic, forexample.

Let us also take a closer look at the reduction of the total systemdelay by means of the method of the invention. Since the interleavinglength t_(i) of the interleaving subsequent to the inner coding can beincreased to equal the interleaving length t_(o) of the interleavingsubsequent to the outer coding, this interleaving length t_(o) can beshortened without impairing the performance, compared with the solutionsof the prior art. Using the above numerical values as an example, let usassume that previously t_(o)=100 ms and t_(i)=10 ms, thus the totaldelay caused by interleaving being 200 ms. In the solution of theinvention, t_(o) and t_(i) can be chosen to be equal in length in such amanner, for instance, that both are given the value t_(o)=t_(i)=80 ms.The total delay caused by interleaving thus becomes 160 ms. Thenumerical values mentioned herein are only used by way of example. Inpractical solutions the values are chosen on the basis of systemparameters.

Even though the invention has been described in the above with referenceto the example in accordance with the accompanying drawings, it is to beunderstood that the invention is not restricted to it, but can bemodified in many ways within the scope of the inventive idea disclosedin the appended claims.

What is claimed is:
 1. A data transmission method in a wirelesstelecommunications system, the method comprising the steps of:transmitting user signals in a given channel within a maximum delaydefined for the system; sequentially performing an outer coding and aninner coding on the signals to be transmitted; performing a firstinterleaving to a signal after the outer coding; choosing theinterleaving length of the first interleaving to be within the scope ofthe maximum delay defined for the system; and performing a secondinterleaving to the signal after the inner coding wherein theinterleaving length of the second interleaving is substantially equal tothe interleaving length of the first interleaving and the signals to betransmitted comprise signals of several quality standards and datarates.
 2. A data transmission method in a wireless telecommunicationssystem, the method comprising steps of: transmitting user signals in agiven channel within a maximum delay defined for the system;sequentially performing an outer coding and an inner coding on thesignals to be transmitted; performing a first interleaving to a signalafter the outer coding; choosing the interleaving length of the firstinterleaving to be within the scope of the maximum delay defined for thesystem; performing a second interleaving to the signal after the innercoding wherein the interleaving length of the second interleaving issubstantially equal to the interleaving length of the first interleavingand the signals to be transmitted include signals of several qualitystandards and data rates; performing the outer coding in such a mannerthat all signals to be transmitted have a common quality standard;combining the signals obtained in this manner having the same qualitystandard into frames of a given length; performing the inner coding tothe combined signal; and repeating or removing some symbols, ifnecessary, in the frames to be transmitted, in such a manner that thelength of the symbols in each frame is the multiple of a time periodthat is substantially shorter than the symbol length.
 3. A datatransmission system wherein the user signals are transmitted wirelesslyin a given channel and a maximum delay has been defined for the system,the system comprising: means in the transmitting end for performingsequentially an outer coding and an inner coding to the signals to betransmitted; wherein the system further comprises means for performingthe outer coding in such a manner that all signals to be transmittedhave a common quality standard; means for combining the signals to betransmitted into frames that have a given length; means for performingthe inner coding to the signals to be transmitted; and means forrepeating or removing some symbols in each frame, if necessary, in sucha manner that the length of the symbols in each frame is equal to themultiple of a time period which is substantially shorter than the symbollength; and means for performing a first interleaving to a signal afterthe outer coding, the interleaving length of the first interleavingbeing chosen within the scope of the maximum delay defined for thesystem, wherein the system further comprises means for performing asecond interleaving to the signal after the inner coding, theinterleaving length of the second interleaving being substantially equalto the interleaving length of the first interleaving.